Seasonal and Diurnal Variability of Monoterpenes in the Eastern Mediterranean Atmosphere

Monoterpenes significantly affect air quality and climate as they participate in tropospheric ozone formation, new particle formation (NPF), and growth through their oxidation products. Vegetation is responsible for most biogenic volatile organic compound (BVOC) emissions released into the atmosphere, yet the contribution of shrub and regional transport to the ambient monoterpene mixing ratios is not sufficiently documented. In this study, we present one-year systematic observations of monoterpenes in the Eastern Mediterranean at a remote coastal site, affected mainly by the typical phrygana vegetation found on the Island of Crete in Greece. A total of 345 air samples were collected in absorption tubes and analyzed by a GC-FID system during three intensive campaigns (in spring 2014, summer 2014, and spring 2015) in addition to the systematic collection of one diurnal cycle per week from October 2014 to April 2015. Limonene, α-pinene and 1,8-cineol have been detected. The mixing ratios of α-pinene during spring and summer show a cycle that is typical for biogenic compounds, with high levels during the night and early morning, followed by an abrupt decrease around midday, which results from the strong photochemical depletion of this compound. Limonene was the most abundant monoterpene, with average mixing ratios of 36.3 ± 66 ppt. The highest mixing ratios were observed during autumn and spring, with a maximum mixing ratio in the early afternoon. The spring and autumn maxima could be attributed to the seasonal behavior of vegetation growth at Finokalia. The green period starts in late autumn when phrygana vegetation grows because of the rainfall; the temperature is still high at this time, as Finokalia is located in the southeast part of Europe. Statistical analyses of the observations showed that limonene and α-pinene have different sources, and none of the studied monoterpenes is correlated with the anthropogenic sources. Finally, the seasonality of the new particle formation (NPF) events and monoterpene mixing ratios show similarities, with a maximum occurring in spring, indicating that monoterpenes may contribute to the production of new particles.